Harris is accused of disorderly behavior during an Oct. 1 march across Brooklyn Bridge. Prosecutors want to use his tweets as evidence that he was "well aware of police instructions" not to block traffic, the BBC explained.

Harris attempted to block the subpoena last month but was denied, after a judge found that he had no legal standing to block access to his Twitter account details and posts. In the ruling, the judge compared the information to bank records, over which the account holder "has no proprietary or possessory interests."

Twitter, however, has disputed that judgment. One of the company's lawyers, Ben Lee, told AFP:

"Twitter's terms of service make absolutely clear that its users 'own' their content. Our filing with the court reaffirms our steadfast commitment to defending those rights for our users."

"One potential problem for free speech on the Internet is that, for almost all of us, we need to rely on Internet companies," wrote ACLU senior staff attorney Aden Fine in a blog post. "And while the government is bound by the First Amendment, the First Amendment may not always prevent private companies from restricting our free speech rights.

"That is why it is so important to encourage those companies that we all increasingly rely on to do what they can to protect their customers' free speech and privacy rights."

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